The electrolytic recovery of nickel has been practiced by the alternative methods known as electrorefining and electrowinning respectively. Electrorefining involves the use of soluble anodes which may consist of metallic nickel or cast nickel matte. Matte anodes are fragile and leave residues upon dissolution, the handling of which contributes significantly to the cost of process. Metallic anodes, on the other hand, have been produced from nickel mattes pyrometallurgically by a roast-reduction treatment which is costly in terms of energy as well as pollution control measures which may be needed.
A more attractive scheme for treating mattes would be to leach the nickel therefrom and then recover it by electrowinning from solution. One such approach that has been advocated involves chloride leaching and electrowinning. Such a process has several disadvantages. Sulfur present in the matte results in hydrogen sulfide in the off-gases and chlorine is evolved during the electrowinning so that environmental problems are created. Moreover, unless prior roasting of the matte is undertaken, it is necessary to resort to concentrated hydrochloric acid and hence to corrosion resisting equipment.
Ammonia leaching has been used to dissolve the nickel from the matte and subsequently recover it. However, inherent difficulties of such a scheme are the environmental objections to ammonia, and the fact that the sulfur present in the matte reports as ammonium sulfate which has to be sold profitably in order for the process to be economically viable.
Sulfuric acid leaching of the matte has been widely studied as a first step to electrowinning the nickel from an all-sulfate solution. However, the cost of providing the acid needed, the corrosiveness of the leach medium and the possible evolution of hydrogen sulfide all detract from the commercial utility of a sulfuric acid leach. More recently two alternative schemes have been proposed to overcome the objections to the use of sulfuric acid. The first of these is described in a paper by: G. Nakazawa, A. Suetsuna and T. Shimogawara, entitled: "Development of the Pressure Oxidation Leaching of High Grade Nickel Matte", presented at the annular meeting of AIME at Chicago, March, 1973. The scheme described involves the addition of elemental sulfur to matte and subsequent leaching with water and oxygen in an autoclave. In this way, sulfuric acid is produced in situ by oxidation of the elemental sulfur added as well as sulfur in the feed. The authors recommend establishing an equiatomic ratio of sulfur to nickel by adding the appropriate amount of sulfur. They state, that the atomic ratio of S:Ni may be as low as 0.6 in which case a basic nickel sulfate is formed which they dissolve in sulfuric acid. However, their experimental results show that when they used an atomic S:Ni ratio of 0.66 the nickel extraction was only of the order of 42%.
A second alternative scheme to the use of sulfuric acid in the autoclave is described in U.S. Pat. No. 3,975,190 issued to Van der Muelen et al. In this case, the matte is slurried in a solution of ammonium sulfate, sodium sulfate or nickel sulfate and treated in an autoclave with oxygen at a pH of 4.5-6.5 to transform the nickel into a basic precipitate which is thereafter separated and then dissolved in sulfuric acid.
A drawback of both the Nakazawa et al. and the Van der Meulen et al. schemes is that neither of them provides a convenient manner for rejecting the sulfur initially present in the feed. In both cases most of the matte sulfur is fed to the electrowinning circuit as sulfate ions together with the nickel. Thus, in the case of the Nakawaza et al. scheme, sulfate corresponding not only to the matte sulfur but also to the added elemental sulfur would be fed to the electrowinning circuit with the nickel. On the other hand, Van der Meulen et al. describe the leach as resulting in a nickel hydroxide precipitate. However, in practice, this would not be the case, rather the nickel would be in the form of a basic nickel sulfate, thus containing a significant amount of the matte sulfur. If the subsequent dissolution of the precipitate is carried out with spent electrowinning electrolyte, the result will be that the electrolyte will not only be replenished with nickel, but also will increase in sulfate concentration. Because the electrowinning operation does not deplete sulfate from the electrolyte, any introduction of sulfate causes a build-up and necessitates a bleed which is costly to perform without introducing undesirable foreign ions into the electrolyte.